Metabolic Diseases & Starvation

🧬 Essential Amino Acids

You get essential amino acids ONLY from diet. Your body cannot synthesize them. No metabolic cycle can create them.

The "Pvt Tim Hall" Mnemonic

10 Essential Amino Acids:

Phenylalanine
valine
threonine
Tryptophan
isoleucine
methionine
Histidine
arginine*
Leucine
lysine

*Arginine is conditionally essential (essential during illness/stress)

Challenge: Deficiency Impact

A deficiency in essential amino acids leads to which of the following?

A) Transient metabolic state with no long-term consequences

B) Muscle disease and starvation state

C) Primarily affects lipid metabolism

D) Results in hyperammonemia only

💡 Non-essential AAs: Your body MAKES them FROM essential AAs. Think of essentials as building blocks and non-essentials as derivatives.

The "Trade" Reactions (Board Favorite!)

Two critical conditional essentiality relationships:

Phenylalanine → Tyrosine (if Phe deficient, Tyr becomes essential)
Methionine → Cysteine (if Met deficient, Cys becomes essential)

During starvation/deficiency: Body breaks down proteins to get amino acids. The primary source? Skeletal muscle — the largest protein reserve in the body.

⏱️ Energy Utilization During Starvation

Board Gold: The body follows a STRICT energy hierarchy. Understanding this timeline is essential for diagnostic reasoning.

Phase 1: Plasma Glucose (2-4 hours)

Immediate energy source. Hormones: Epinephrine + Glucagon drive mobilization. Insulin peaks at 2-4 hours.

Phase 2: Liver Glycogen (24-28 hours)

By end of day 2, glucose stores depleted. Hormones: Epi + Glucagon maintain mobilization.

Phase 3: Proteolysis for Gluconeogenesis (starts day 2)

Amino acids converted to glucose. Muscle mass decreases. New hormones: Cortisol + GH (after 24h).

Phase 4: Lipolysis (starts end of day 2)

Fat breakdown. Body holds onto fats longest — they're the most efficient energy store. Hormone: Glucagon.

Phase 5: Ketogenesis (as needed)

Lipids → ketones. Only occurs if needed. Hormone: Glucagon. Provides alternative fuel for brain.

🔑 The body ALWAYS follows this order. It's hardwired. You cannot skip phases or rearrange them.

Diagnostic Reasoning from Biomarkers

Low glucose + high glycogen → Problem mobilizing glycogen (genetic defect)
Low glucose + low glycogen + high lipids/ketones → Problem with proteolysis
Low glucose + low glycogen + decreased muscle + low lipids/ketones → Problem breaking down lipids
Low sugar always hurts RBCs first — they can't switch fuel sources

Malnutrition Types

Kwashiorkor: Protein-calorie malnutrition (adequate calories, no protein)
Marasmus: Calorie deficiency (inadequate total calories)

Challenge: Diagnostic Pattern

A patient presents with low plasma glucose, depleted liver glycogen, elevated plasma ketones, and decreased skeletal muscle mass. What is the PRIMARY metabolic problem?

A) Impaired glycogenolysis

B) Impaired lipolysis (can't break down fats effectively)

C) Impaired proteolysis

D) Normal starvation — all patterns are expected

Clinical Pearl: Newborn screening at 2 days looks for manifestations of protein breakdown disorders. Early detection saves lives.

🧪 PKU (Phenylketonuria)

Board LOVES this. Autosomal recessive inheritance. Deficient enzyme: Phenylalanine hydroxylase

The Three-Part Catastrophe

1. Can't make Tyrosine

→ Can't make dopamine, norepinephrine, epinephrine
→ Intellectual disability

2. Can't make Melanin

→ Blonde hair, blue eyes, fair skin (pale phenotype)

3. Phenylacetate & Phenylpyruvate Accumulate

→ Toxic to brain
→ Musty/mouse-like odor (characteristic smell)

Screening & Treatment

Screen at 48 hours (Guthrie test — detects phenylacetate & phenylpyruvate)
Must avoid aspartame (NutraSweet) — contains phenylalanine
Pregnant mother with PKU: Strict restricted diet, especially first 8 weeks
Screening increases BOTH incidence AND prevalence (because we catch more cases)

💡 The pale, blonde, blue-eyed baby with a musty odor who screams? Think PKU. It's a clinical triad + odor.

Challenge: Maternal PKU

A woman with PKU on a phenylalanine-restricted diet becomes pregnant. Why is strict dietary compliance especially critical during the first 8 weeks?

A) To prevent PKU in the fetus (genetic status is fixed)

B) High maternal phenylalanine damages fetal CNS development during organogenesis

C) To prevent hyperammonemia in the mother

D) To maintain adequate cofactor availability

🔬 Tyrosine Catabolism Pathway

Interactive decision tree: Where are blocks in this pathway clinically important?

Phenylalanine

↓ (1. Phenylalanine hydroxylase + BH₄)

Tyrosine

↙ (2. Tyrosine hydroxylase + BH₄) ↓ (4. Tyrosinase) ↘

DOPA

↓ (3. DOPA decarboxylase + B₆)

Dopamine
(NE, Epi)

Melanin

Clinical Blocks

Block at Step 1 (Phe hydroxylase deficiency) = PKU
Block at Step 4 (Tyrosinase deficiency) = Albinism

Challenge: Pathway Block Logic

In albinism, which product is NOT decreased?

A) Dopamine and catecholamines remain normal

B) Melanin is decreased

C) Both melanin and dopamine are absent

D) DOPA levels are decreased

🎨 Pigment Disorders

Albinism

Tyrosinase deficiency → No melanin production

Massive skin cancer risk (melanocytes missing = no protection)
Minimum SPF 30 required (blocks 96% of UVA)

Vitiligo

Autoimmune attack on melanocytes (antibodies against melanocyte antigens)

Loss of skin pigmentation (depigmented patches)
Starts on face — characteristic presentation
Skin cancer risk (like albinism)

Alkaptonuria (Ochronosis)

Homogentisic acid oxidase deficiency → Backup accumulation

Urine turns BLACK in air (oxidized homogentisic acid)
Tyrosine backs up into melanin pathway
Black nasal septum, tendons, earlobe (ochronotic deposits)

💡 Alkaptonuria Classic Triad: Black urine + black nose + black ears. It's literally painting the patient dark. The baby's diaper turns black.

Challenge: Pigment Disorder Differentiation

Which pigment disorder features autoimmune destruction of melanocytes AND starts on the face?

A) Albinism

B) Vitiligo

C) Alkaptonuria

D) Phenylketonuria

⚗️ Other Key Metabolic Diseases

Maple Syrup Urine Disease (MSUD)

Affected amino acids: BCAAs (Branched-Chain Amino Acids): Leucine, Isoleucine, Valine

Defect: Defective renal transport in collecting duct

Characteristic finding: Urine smells like maple syrup or burnt sugar

Why? Accumulated branched-chain keto acids create the distinctive odor.

Cystinuria

Affected amino acids: Cysteine, Ornithine, Lysine, Arginine (COLA mnemonic)

Defect: Defective renal transport

Characteristic finding: Hexagonal/envelope/coffin-lid shaped crystals in urine

Clinical significance: Cystine stones → risk for chronic kidney disease

💡 COLA mnemonic: Cysteine, Ornithine, Lysine, Arginine. These four amino acids share the same renal transporter. When it's broken, all four spill into urine.

Challenge: Amino Acid Transport Disorders

A newborn presents with hexagonal crystals in the urine. Which amino acids are likely being wasted?

A) Cysteine, Ornithine, Lysine, Arginine

B) Leucine, Isoleucine, Valine

C) Phenylalanine only

D) Glycine and Serine

👶 Newborn Screening Panel

Critical metabolic and endocrine disorders screened at 24-48 hours of life. Early detection prevents death or disability.

PKU (Guthrie Test)

Detects phenylacetate & phenylpyruvate. Screen at 48h.

Hypothyroidism (TSH)

Elevated TSH → primary hypothyroidism. Prevents cretinism.

CAH (Congenital Adrenal Hyperplasia)

Elevated 17-hydroxyprogesterone. 21-hydroxylase deficiency most common.

Biotinidase Deficiency

Treatment: 10 mg biotin daily. Simple but life-saving.

Galactosemia

Lactose intolerance + cataracts. Screen with galactose-1-phosphate uridyltransferase.

Celiac Disease

Tissue transglutaminase (tTG) antibodies. Screen via IgA measurement.

Cystic Fibrosis

Immunoreactive trypsin (IRT). Pancreatic dysfunction marker.

Challenge: Newborn Screening

Which newborn screening condition is treated with a simple vitamin supplement?

A) Hypothyroidism

B) Biotinidase deficiency (10 mg biotin daily)

C) PKU

D) Galactosemia

🧠 Randomized Challenge Quiz

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